The long-term objective is to reveal neuromuscular mechanisms for controlling complex movements. The previous grant cycle focused on the processes of shaping the hand and making movement sequences. The results showed that motor units, not muscles, are the functional elements, and that the control of movement sequences is continuous, not segmented. This renewal application aims to study the sensorimotor control of object manipulation, i.e., the control of arm and hand movements that involve substantial contact forces. Specific Aim 1 will test for a separation of transport and grasp components of arm and hand muscle activation. Based on prior evidence, it is thought that the arm and hand have distinct control mechanisms, with the arm responsible for transport and the hand solely responsible for grasping. Electromyographic (EMG) patterns from multiple muscles will be related to the direction of transport and the level of grasp stability. Preliminary studies suggest that the results will reveal an activation gradient from proximal to distal muscles, rather than distinct control mechanisms for the arm and hand. Specific Aim 2 will examine the efficacy of reflexive adjustments to the balance of hand muscle activation, during simple manipulation tasks. Since somatosensory feedback is generally thought to trigger new motor commands, its possible role in fine-tuning ongoing commands is largely unknown. The proposed experiments are designed to test for a significant change in hand muscle directional tuning, due to the directional characteristics of a mild, unexpected frictional resistance or force field. Specific Aim 3 will characterize the influence of joint positions and contact forces on the perception of hand shape. Bimanual matching experiments will test 1) whether a discrete somatosensory distortion influences whole hand shape, and 2) whether the perception of hand shape is invariant to the level of contact force. The results will provide new information on neuromuscular control strategies for object manipulation and will determine the coordinate system in which multidimensional somatosensory information influences hand shape. These outcomes will be directly relevant to the design of prosthetic interfaces for restoring hand function and to the design of haptic interfaces (as in remote surgical operations). The results will also improve the scientific foundation for inventing treatments for focal hand dystonia, a disease with a complex and poorly understood somatosensory and motor basis.